The Columbus Attached Pressurised Module (APM) Active Thermal Control System (ATCS) water loop collects the APM waste heat and transfers it to the Space Station Freedom (SSF) Central Thermal Bus (CTB). The interface between the APM water loop and the SSF ammonia loops is achieved with two ammonia/water interloop heat exchangers (IH/X), one being low temperature (LT) and the other moderate temperature (MT).The APM internal water loop provides cooling to payload and subsystem users which have varying temperature requirements at their heat rejection interfaces, and can be categorized as cold branch and warm branch users, (e.g. condensing heat exchanger (CHX) and refrigerator are cold branch users, while Avionic heat exchanger (AHX) and furnace payloads would be warm branch users.)New SSF IH/X interface requirements have been introduced in order to optimise the overall SSF heat collection and rejection resources, and as a result of evolving IH/X and CTB design definition following the SSF restructuring. These requirements limit the amount of warm branch heat load which can be transferred to the SSF LT IH/X, limit the allowed flowrate in each IH/X and provide a wider range of ammonia loop temperatures at each IH/X.This paper describes the APM ATCS tradeoffs which have been performed in order to arrive at an optimised water loop architecture able to meet all existing requirements and the new SSF IH/X interface requirements.